Chorda tympani
The chorda tympani is a nerve that branches from the facial nerve (cranial nerve VII) inside the facial canal, just before the facial nerve exits the skull via the stylomastoid foramen. It serves the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain. The chorda tympani is part of one of three cranial nerves that are involved in taste. The taste system involves a complicated feedback loop, with each nerve acting to inhibit the signals of other nerves. The chorda tympani appears to exert a particularly strong inhibitory influence on other taste nerves, as well as on pain fibers in the tongue. When the chorda tympani is damaged, its inhibitory function is disrupted, leading to less inhibited activity in the other nerves. Nerve fibers The chorda tympani carries two types of nerve fibers from their origin with the facial nerve to the lingual nerve that carries them to their destinations: *Special sensory fibers providing taste sensation from the anterior two-thirds of the tongue. *Presynaptic parasympathetic fibers to the submandibular ganglion, providing secretomotor innervation to two salivary glands: the submandibular gland and sublingual gland and to the vessels of the tongue, which when stimulated, cause a dilation of blood vessels of the tongue. Path Exits the skull through the internal acoustic meatus as part of the facial nerve, then it travels through the middle ear, where it runs from posterior to anterior across the tympanic membrane. It passes between the malleus and the incus, on the medial surface of the neck of the malleus. The nerve continues through the petrotympanic fissure, after which it emerges from the skull into the infratemporal fossa. It soon combines with the larger lingual nerve, a branch of the mandibular nerve The fibers of the chorda tympani travel with the lingual nerve to the submandibular ganglion. Here, the preganglionic fibers of the chorda tympani synapse with postganglionic fibers which go on to innervate the submandibular and sublingual salivary glands. Special sensory (taste) fibers also extend from the chorda tympani to the anterior 2/3 of the tongue via the lingual nerve. Research Tastes There are similarities between the tastes the chorda tympani picks up in sweeteners between mice and primates, but not rats. Relating research results to humans is therefore not always consistent.Golden, G. J., Ishiwatari, Y., Theodorides, M. L., & Bachmanov, A. A. (2011). Effect of chorda tympani nerve transection on salt taste perception in mice. Chemical Senses, 36 (9), 811-819. doi: 10.1093/chemse/bjr056 Sodium chloride is detected and recognized most by the chorda tympani nerve. Golden, G. J., Ishiwatari, Y., Theodorides, M. L., & Bachmanov, A. A. (2011). Effect of chorda tympani nerve transection on salt taste perception in mice. Chemical Senses, 36 (9), 811-819. doi: 10.1093/chemse/bjr056 The recognition and responses to sodium chloride in the chorda tympani is mediated by amiloride-sensitive sodium channels. Sollars, S. I., & Bernstein, I. L. (1994). Amiloride sensitivity in the neonatal rat. Behavioral Neuroscience, 108 (5), 981-987. The chorda tympani has a relatively low response to quinine and varied responses to hydrochloride. The chorda tympani is less responsive to sucrose than the greater superficial petrosal nerve.Sollars, S.I., & Hill, D. L. (2005). In vivo recordings from rat geniculate ganglia: taste response properties of individual greater superficial petrosal and chorda tympani neurones. The Journal of Physiology, 564 (3), 877-893. doi:10.1113/jphysiol.2005.083741 Chorda tympani transection The chorda tympani nerve carries its information to the nucleus of the solitary tract, and shares this area with the greater superficial petrosal and glossopharyngeal nerves. When the greater superficial petrosal and glossopharyngeal nerves are cut, regardless of age, the chorda tympani nerve takes over the space in the terminal field. This takeover of space by the chorda tympani is believed to be the nerve reverting to its original state before competition and pruning. Corson, S. L., & Hill, D. L. (2011). Chorda tympani nerve terminal field maturation and maintenance is severely altered following changes to gustatory nerve input to the nucleus of the solitary tract. The Journal of Neuroscience, 31(21), 7591-7603. doi:10.1523/JNEUROSCI-11.2011 The chorda tympani, as part of the peripheral nervous system, is not as plastic in early ages. In a study done by Hosley et al. and a study done by Sollars, it has been shown that when the nerve is cut at a young age, the related taste buds are not likely to grow back to full strength. Hosley, M. A., Hughes, S. E., Morton, L. L., & Oakley, B. (1987). A sensitive period for the neural induction of taste buds. The Journal of Neuroscience, 7, 2075-2080. Sollars, S. I. (2005). Chorda tympani nerve transaction at different developmental ages produces differential effects of taste bud volume and papillae morphology in the rat. The Journal of Neurobiology, 64, 310-320. In a bilateral transection of the chorda tympani in mice, the preference for sodium chloride increases compared to before the transection. Also avoidance of higher concentrations of sodium chloride is eliminated. Golden, G. J., Ishiwatari, Y., Theodorides, M. L., & Bachmanov, A. A. (2011). Effect of chorda tympani nerve transaction on salt taste perception in mice. Chemical Senses, 36 (9), 811-819. doi: 10.1093/chemse/bjr056 The amiloride-sensitive channels responsible for salt recognition and response is functional in adult rats but not neonatal rats. This explains part of the change in preference of sodium chloride after a chorda tympani transection. Sollars, S. I., & Bernstein, I. L. (1994). Amiloride sensitivity in the neonatal rat. Behavioral Neuroscience, 108 (5), 981-987. The chorda tympani innervates the fungiform papillae on the tongue. Sollars, S. I. (2005). Chorda tympani nerve transection at different developmental ages produces differential effects of taste bud volume and papillae morphology in the rat. The Journal of Neurobiology, 64, 310-320. According to a study done by Sollars et al. in 2002, when the chorda tympani has been transected early in postnatal development some of the fungiform papillae undergo a structural change to become more “filliform-like”. Sollars, S. I., Smith, P. C., & Hill, D. L. (2002). Time course of morphological alterations of fungiform papillae and taste buds following chorda tympani transection in neonatal rats. The Journal of Neurobiology, 51 (3), 223-236. doi:10.1002/neu.10055 When some of the other papillae grow back, they do so without a pore. Sollars, S. I. (2005). Chorda tympani nerve transection at different developmental ages produces differential effects of taste bud volume and papillae morphology in the rat. The Journal of Neurobiology, 64, 310-320. Additional images Image:Gray178.png|Mandible of human embryo 24 mm. long. Outer aspect. Image:Gray778.png|Distribution of the maxillary and mandibular nerves, and the submaxillary ganglion. Image:Gray788.png|Plan of the facial and intermediate nerves and their communication with other nerves. Image:Gray789.png|The course and connections of the facial nerve in the temporal bone. Image:Gray842.png|Sympathetic connections of the submaxillary and superior cervical ganglia. Image:Gray911.png|View of the inner wall of the tympanum (enlarged.) Image:Chorda tympani nerve 2.jpg|Dissection of chorda tympani nerve File:Slide1CAC.JPG|Lateral head anatomy detail.Facial nerve dissection. References and notes External links * * * * * ( ) * Photo at Washington University Category:Facial nerve Category:Ear